Physical body

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Physical body

In physics, a physical body or physical object (sometimes simply called a body or object) is an identifiable collection of matter, which may be more or less constrained to move together by translation or rotation, in 3-dimensional space.

In common usage an object is a collection of matter within a defined contiguous boundary in 3-dimensional space. The boundary must be defined and identified by the properties of the material. The boundary may change over time. The boundary is usually the visible or tangible surface of the object. The matter in the object is constrained (to a greater or lesser degree) to move as one object. The boundary may move in space relative to other objects that it is not attached to (through translation and rotation). An objects boundary may also deform and change over time in other ways.

Also in common usage an object is not constrained to consist of the same collection of matter. Atoms or parts of an object may change over time. An object is defined by the simplest representation of the boundary consistent with the observations. However the laws of Physics only apply directly to objects that consist of the same collection of matter.

Each object has a unique identity, independent of any other properties. Two objects may be identical, in all properties except position, but still remain distinguishable. In most cases the boundaries of two objects may not overlap at any point in time. The property of identity allows objects to be counted.

A body with known composition and described in an adequate physical theory is an example of physical system.

Contents

In common usage1

Classical mechanics2

Quantum mechanics3

String theory4

In psychology5

In philosophy6

In new age philosophy, mysticism and religion7

See also8

External links9

In common usage

An object is known by the application of senses. The properties of an object are inferred by learning and reasoning based on the information perceived. Abstractly, an object is a construction of our mind consistent with the information provided by our senses, using Occam's razor.

In common usage an object is the material inside the boundary of an object, in 3-dimensional space. The boundary of an object is a contiguous surface which may be used to determine what is inside, and what is outside an object. An object is a single piece of material, whose extent is determined by a description based on the properties of the material. An imaginary sphere of granite within a larger block of granite would not be considered an identifiable object, in common usage. A fossilized scull encased in a rock may be considered an object because it is possible to determine the extent of the scull based on the properties of the material.

An object has an identity. In general two objects with identical properties other than position at an instance in time may be distinguished as two objects may not occupy the same space at the same time (excluding component objects). An objects identity may be tracked using the continuity of the change in its boundary over time. The identity of objects allows objects to be arranged in sets and counted.

The material in an object may change over time. For example, a rock may wear away or have pieces broken off it. The object will be regarded as the same object after the addition or removal of material, if the system may be more simply described with the continued existence of the object, than in any other way. The addition or removal of material may discontinuously change the boundary of the object. The continuation of the objects identity is then based on the description of the system by continued identify being simpler than without continued identity.

For example, a particular car might have all its wheels changed, and still be regarded as the same car.

The identity of an object may not split. If an object is broken into two pieces at most one of the pieces has the same identity. An objects identity may also be destroyed if the simplest description of the system at a point in time changes from identifying the object to not identifying it. Also an objects identity is created at the first point in time that the simplest model of the system consistent with perception identifies it.

An object may be composed of components. A component is an object completely within the boundary of a containing object.

Under Newtonian gravity the gravitational field further away than the furthest extent of an object is determined only by the mass and the position of the center of mass.

Interactions between objects are partly described by orientation and external shape.

In continuum mechanics an object may be described as a collection of sub objects, down to an infinitesimal division, which interact with each other by forces which may be described internally by pressure and mechanical stress.

These ideas vary from the common usage understanding of what an object is. Object/particle identity is shared between common usage and quantum mechanics, although in quantum mechanics some particles may occupy the same position at the same time (e.g. photons).

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